scripts: sources-branch-updater: Make regexp for master branch stricter
scripts: sources-branch-updater: Make regexp for master branch stricter

The regexp does not work properly when the upstream repo has many
branches named 'master' such as ceph-ansible

0fa368a7441528457b7bcb99f8a860d7987b9ba6 refs/heads/guits-update_req_master
79bd06ad28d8a078bd95923b230eb8d5b66095b2 refs/heads/master

which breaks the script when bumping SHAs. As such, lets make it
stricter by simply matching /master$ instead.

Change-Id: I24526a358d7a21821f8c9789ba199863981f8d7b

stackforge/os-ansible-deployment,openstack/openstack-ansible,openstack/openstack-ansible,stackforge/os-ansible-deployment